Catalysts are employed in the exhaust systems of automotive vehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides (NOx) produced during engine operation into nonpolluting gases. When the gasoline powered engine is operated in a stoichiometric or slightly rich air/fuel ratio, i.e., between about 14.7 and 14.4, catalysts containing precious metals like palladium and rhodium are able to efficiently convert all three gases simultaneously. Hence, such catalysts are often called "three-way" catalysts.
Engines operating on diesel fuel are commonly used in Europe and are known for their enhanced fuel economy relative to gasoline engines. Diesel engines operate under oxygen-rich (often termed: lean-burn) conditions where the A/F ratio is greater than 19, generally 19-40 and under relatively cool operating temperatures as compared to gasoline engines. Three-way catalysts developed for treatment of exhaust gas from conventional gasoline engine operation under stoichiometric A/F ratios are less than desirable for diesel exhaust gas treatment. Such catalysts are able to convert carbon monoxide and hydrocarbons in lean-burn operation but are not efficient in the reduction of NOx (NO+NO.sub.2) during diesel operation due to the lower temperature and excess oxygen. The desire for efficient diesel catalysts to meet European upcoming diesel emission standards continues to prompt research. Due to the nature of diesel engine exhaust, these catalysts must be able to reduce NOx at relatively low temperatures in an oxygen rich environment.
Available diesel catalysts are often based on zeolite materials containing a precious metal like platinum which can have major drawbacks. Among the most important are a narrow temperature range of operation and loss of activity (and sometimes physical integrity) under the hydrothermal conditions of automotive exhaust gases. For example, a platinum catalyst is generally only active at a relatively low temperature, i.e., less than 250.degree. C. At higher temperatures the competitive oxidation of the reductant hydrocarbon molecules by oxygen is so fast that the removal of NOx drops off precipitously with rising temperature so as to make such catalyst inadequate for treating somewhat hotter exhaust streams. On the other hand, a different type of catalyst where the active sites are transition metal ions exchanged into the cationic sites of the zeolite, the onset of selective catalyst reduction activity begins at temperatures greater than 400.degree. C. This renders the ion-exchanged catalyst inactive for catalysis during a large portion of the desired temperature range. Other diesel catalysts are based on support materials such as silica, gamma-alumina, titanium oxide, zirconium oxide or some combination thereof. These catalysts have the drawback, however, that they either have rather low NOx conversion efficiencies or have narrow NOx conversion temperature windows.
As discussed above, platinum impregnated alumina materials are considered viable candidates for the aftertreatment of diesel exhaust at low temperatures. Such catalysts act to reduce the NOx through the use of hydrocarbons over a catalyst, the hydrocarbons being in turn oxidized. However, such catalysts typically have a rather narrow NOx conversion temperature window. We have now unexpectedly found that a catalyst which is a physical mixture of manganese/zirconium oxide with platinum/alumina provides a wider NOx conversion window in the low temperature region. In U.S. application Ser. no. 09/134,992 filed Aug. 17, 1998 commonly assigned herewith, a NOx trap catalyst for lean-burn engines is disclosed. It comprises platinum with a tri-metal oxide of aluminum oxide, manganese oxide, and zirconium oxide made by sol-gel techniques.